The present invention relates to a cellular communication system, and more particularly to a cellular communication system, wherein neighboring cells share antennas so as to create an overlapping region which is covered by at least two cells.
In a cellular mobile radio telephone system, a collection of cells or radio base stations each having a specified coverage area is controlled to provide duplex radio communications with a large number of mobile stations throughout a geographical area. A typical cellular communication system is illustrated in FIG. 1. FIG. 1 illustrates ten cells, C1-C10, in a typical cellular mobile radio communication system. Normally, a cellular mobile radio system will be implemented with more than ten cells. However, for the purposes of simplicity, the present invention can be explained using the simplified representation illustrated in Figure. 1. For each cell, C1-C10, there is a base station B1-B10, with the same reference number as a corresponding cell. FIG. 1 illustrates the base stations as situated in the vicinity of the cell center and having omni-directional antennas. The base stations could also be situated at a cell border and use directional antennas. Each of the base stations is connected to a base station controller BSC which controls some of the operations of the base station.
FIG. 1 also illustrates nine mobile stations, M1-M9 which are movable within a cell and from one cell to another. In a typical cellular radio system, there would normally be more than nine cellular mobile stations. In fact, there are typically many times the number of mobile stations as there are base stations. However, for the purposes of explaining the present invention, the reduced number of mobile stations is sufficient.
Also illustrated in FIG. 1 is a mobile services switching center MSC. The mobile services switching center MSC is connected to all of the base station controllers by cables. The mobile service switching center MSC is also connected by cables to a fixed switch telephone network or similar fixed network. All cables from the mobile services switching center MSC to the base station B1-B10 and cables to the fixed network are not illustrated.
In addition to the mobile services switching center MSC illustrated, there may be additional mobile switching centers connected by cables to base stations other than those illustrated in FIG. 1. Instead of cables, other means, for example, fixed radio links, may also be used between base stations, base station controllers and mobile services switching centers. The mobile services switching center MSC, the base stations, and the mobile stations are all computer controlled.
As noted above, cellular communication systems are built by using base stations or base transceiver stations. The mobile stations can be connected by electromagnetic radio waves to the base stations. For the cellular system to function in the desired way, it is important that mobiles can be handed off or handed over between base stations or cells. As a result, during a call, the connection between a mobile station and another party is switched from one base station to another as the mobile station travels through the system.
In order for the cellular system to be able to decide which base station a mobile station should be connected to, the mobile station and/or the cellular system performs measurements on received signals such as signal strength, path loss, etc. Either the mobile station can measure the signal strength of signals received from the base stations or the base stations can measure the signal strength of signals received from the mobile stations. Regardless of how the signal strength measurements are collected, the data is processed in a mobile services switching center or base station controller in order to decide if a handover should be performed. In some systems, delays are introduced by signalling, e.g., measurement reports and handover commands, between both the mobile stations and the base stations and between the base stations and the processing unit of the cellular system. As a result, the handover procedure must take into account the delay inherent during signal transfers and signal processing.
In an attempt to provide seamless handoffs between cells, overlapping regions of coverage are created between neighboring cells. FIG. 2 illustrates an overlapping region 20 created between cell 22 and cell 24. The overlapping region is defined by points 26 and 28, which indicate where the signal strength of cells 22 and 24, respectively, falls below a predetermined level. However, the overlapping region between cells is sometimes very small. This can be because the radio signal strength from the base station that the mobile station is connected to drops off quickly when the mobile moves away from the base station. In addition, the signal strength from the base station that a mobile station should change to may raise very fast when the mobile station is travelling toward the base station. This phenomena can occur when a mobile station which is in line of sight with its current base station moves around a street corner and is now in line of sight with a new base station. Another example is when a mobile station moves through a tunnel wherein the regions on either side of the tunnel are connected to different base stations. A third example is when a hand held mobile is carried through a door into an office building which has its own cellular system. The common problem in all of the above examples is that the overlapping regions between the cells is so small that when the mobile moves through the overlapping region before the call can be handed over because of the time delays described above with respect to signalling and signal processing are too large. As a result, the call will be lost.
Prior art systems have attempted to improve the speed in which a hand off takes place by reducing the number of neighbors a mobile station measures and by using short filters. However, taking these actions may have a bad influence on the overall performance of the system, since using too few neighbors may forbid a handover between cells for which a handover should be allowed and too short a filter may cause handovers that should not be performed in the first place. Thus, there is a need in the industry to improve the handoff capabilities of a cellular system.
It is an object of the present invention to overcome the problems cited above, by providing a cellular communication system wherein cells within the communication system share at least one transceiver unit or have transceivers placed in close proximity to each other so as to create a larger overlapping region in which mobile stations can be handed off. According to one embodiment of the present invention, the cellular communication system comprises a plurality of cells wherein each cell is assigned a communication frequency and contains a plurality of transceiver units and at least one base station for transmitting and receiving radio signals on the assigned frequency. Neighboring cells in the system share at least one transceiver unit to create an overlapping region wherein the shared transceiver unit is able to transmit and receive radio signals with the communication frequencies of the cells sharing the transceiver unit.
According to another embodiment of the present invention, a method for handing over a call in a cellular system is disclosed, wherein neighboring cells within the communication system share at least one transceiver means to create an overlapping region. Each mobile station receives a list of frequencies used by neighboring cells. Each transceiver means within each cell periodically broadcasts a base station identification code which is unique for each cell within the cellular system. However, the shared transceiver means broadcasts the identification codes for the cells sharing the transceiver means. The mobile stations within the cellular system periodically measure the signal strength of signals received from neighboring cells and then decode any identification codes they receive. The mobile stations then report any decoded information codes and the signal strength measurements for the neighboring cell identified by the decoded identification code to a base station controller. The base station controller then determines when a mobile station should be handed off to a second cell based upon the signal strength measurements.
According to another embodiment of the present invention, a method for handing over a call in a cellular communication system is disclosed. The cellular system is divided into a plurality of cells, each cell having a plurality of transceiver means, wherein neighboring cells share at least one of said transceiver means to create an overlapping region. The shared transceiver means is able to transmit and receive radio signals on frequencies assigned to the cells sharing the transceiver means. According to the present invention, control signals are periodically broadcast from each transceiver means within each cell, wherein the shared transceiver means broadcast control signals of the cells sharing the transceiver means. The signal strength of control signals received at a mobile station are then measured and reported to a base station controller. A determination is then made as to whether mobile station should be handed off to a second cell based on the signal strength measurements.
According to another embodiment of the present invention, a method for handing over a call in a cellular communication system is disclosed. Control signals are periodically broadcast from each transceiver means within each cell, wherein shared transceiver means broadcast control signals of the cells sharing the transceiver means. The signal strength of control signals received at a mobile station are measured and evaluated to determine whether the mobile station should be handed off to a second cell using a handover algorithm and the signal strength measurements at the mobile station. A handover request can then be made to a base station controller based upon the evaluation. The base station controller then determines whether the handover request should be granted based upon system information and hands over the mobile station to the second cell when the handover request is granted.